1. Field of the Invention
The present invention relates to a reception quality measuring apparatus and a reception quality measuring method for measuring the reception quality of a single carrier signal in a mobile communication system, and more particularly, to a reception quality measuring apparatus and a reception quality measuring method which convert a pilot signal included in a single carrier signal from a signal in a time domain to a signal in a frequency domain, and measure the reception quality of the single carrier signal based on an equalized signal after equalization is performed.
2. Description of the Related Art
In an uplink radio system of next-generation mobile communications, a single-carrier (SC) system is regarded as promising for expanding communication areas because of its low peak to average power ratio (PAPR).
Also, in the next-generation mobile communications, the reception quality must be measured at a base station in order to perform scheduling, adaptive modulation and coding (AMC), and transmit power control (TPC) of packet signals. As the reception quality, generally, a signal to interference power ratio (SIR) is measured using a pilot signal (see, for example, Patent Documents 1, 2). Here, the interference power includes noise power.
Patent Document 1: JP-2005-057673-A
Patent Document 2: JP-2006-287754-A
In the SC system, in order to restrain inter-symbol interference (multi-path interference) due to multi-paths, multi-path equalization must be performed at a receiver. As the multi-path equalization, frequency domain equalization, which requires a less amount of processing, is performed.
When the frequency domain equalization is performed in a receiver, SIR differs before equalization and after equalization, and SIR after equalization reflects the correct reception quality. SIR before equalization indicates set SIR independently of multi-path transmission paths, whereas SIR after equalization reflects the influence of the multi-path transmission paths and equalization processing, so that as multi-path conditions become more strict (as the number of paths increases), SIR deteriorates due to noise amplification caused by the equalization and residual multi-path interference. Accordingly, when measured SIR is used for AMC modulation scheme and coding rate selection and TPC, more correct control can be conducted using SIR after equalization.
In the past, SIR after equalization of the SC system is measured in a time domain after an equalized signal in a frequency domain is inverse discrete Fourier transformed (IDFT) for conversion to a signal in the time domain, so that the amount of processing is increased in order to obtain inverse discrete Fourier transformation of the equalized signal of the pilot signal. In order to reduce the amount of processing for IDFT, SIR is preferably measured in the frequency domain.
FIG. 1 shows an exemplary configuration of a receiver for demodulating an SC signal. This receiver measures SIR before equalization or SIR after equalization of the SC signal in the frequency domain.
The receiver shown in FIG. 1 comprises reception antenna 1, CP (Cyclic Prefix) remover 2, discrete Fourier Transform (DFT) unit 3, reception filter 4, channel estimator 5, weight calculation unit 6, frequency domain equalizer 7, IDFT unit 8, and reception quality measuring apparatus 9.
Reception antenna 1 receives the SC signal as a reception signal. FIGS. 2A and 2B show an example of a format for the SC signal.
Upon transmitting a data signal, as shown in FIG. 2A, a data signal is transmitted together with a pilot signal associated therewith for demodulation in the same band as the data signal. The data signal is processed by reception antenna 1, CP remover 2, DFT unit 3, reception filter 4, frequency domain equalizer 7, and IDFT unit 8, and is output as a demodulated signal. The pilot signal for demodulation is processed by reception antenna 1, CP remover 2, DFT unit 3, and reception filter 4, and is applied to channel estimator 5, frequency domain equalizer 7, and reception quality measuring apparatus 9. Specifically, upon transmitting the data signal, reception quality measuring apparatus 9 measures SIR before equalization using the output of channel estimator 5 or the output of reception filter 4, or measures SIR after equalization using equalization weight found based on an estimate of channel gain by weight calculation unit 6, and also based on the output of frequency domain equalizer 7.
On the other hand, when no data signal is transmitted, as shown in FIG. 2B, a pilot signal for probing in an arbitrary band is transmitted at a predetermined period. The pilot signal for probing is processed by reception antenna 1, CP remover 2, DFT unit 3, and reception filter 4, and applied to channel estimator 5 and reception quality measuring apparatus 9. Specifically, when no data signal is transmitted, reception quality measuring apparatus 9 measures SIR before equalization using the output of channel estimator 5 and the output of reception filter 4.
CP remover 2 removes a signal of a portion corresponding to CP from an SC signal received by reception antenna 1.
DFT unit 3 performs DFT of the signal output from CP remover 2 at NDFT points (NDFT is an integer equal to or larger than two) for conversion to a signal in a frequency domain.
Reception filter 4 limits the band of the signal in the frequency domain output from DFT unit 3 to perform user separation and noise suppression. In this regard, while a raised cosine roll off filter is generally used for reception filter 4, a sub-carrier corresponding to the signal band may be selected (demapped) when the roll off rate is zero.
Channel estimator 5 performs correlation processing of the pilot signal in the frequency domain output from reception filter 4 and a pilot reference signal to find a correlation signal (channel gain before noise suppression), and further performs noise suppression to estimate the channel gain. When a pilot code having fixed amplitude characteristics is used in this correlation processing, channel gain H′(k) (1≦k≦K, where K is the number of sub-carriers in the signal band), before the noise suppression, is represented by the following Equation (1):[Equation (1)]H′(k)=R(k)C*(k)  (1)where R(k) is a reception signal in the frequency domain (1≦k≦K), and C(k) is the pilot coding characteristic (1≦k≦K).
Accordingly, channel estimator 5 performs the noise suppression for the foregoing H′(k) to find channel gain H(k).
Weight calculation unit 6 calculates an equalization weight based on the channel gain output from channel estimator 5. As a weight calculation method, a zero forcing method (ZF), or a minimum mean squared error method (MMSE) is used.
Frequency domain equalizer 7 multiplies an equalization weight output from weight calculation unit 6 by a signal in the frequency domain output from reception filter 4 on a sub-carrier by sub-carrier basis to perform multi-path equalization for the reception signal in the frequency domain.
IDFT unit 8 performs IDFT at NIDFT points (NIDFT is an integer equal to or larger than two) for the equalized signal in the frequency domain output from frequency domain equalizer 7 for conversion to a signal in the time domain, and outputs a demodulated signal.
Reception quality measuring apparatus 9 measures SIR before equalization or measures SIR after equalization in the frequency domain using the output of reception filter 4 or the output of frequency domain equalizer 7.
FIG. 3 shows an exemplary configuration of a conventional reception quality measuring apparatus which is incorporated in the receiver shown in FIG. 1 as reception quality measuring apparatus 9. This reception quality measuring apparatus measures SIR before equalization.
The conventional reception quality measuring apparatus shown in FIG. 3 comprises first power calculation unit 112, first sub-carrier averaging unit 113, pilot signal replica generator 114, subtractor 115, second power calculation unit 116, second sub-carrier averaging unit 117, and divider 118.
In the following, a description will be given of the operation of the conventional reception quality measuring apparatus shown in FIG. 3.
First, first power calculating unit 112 calculates the power of channel gain H(k) in the frequency region found in channel estimator 5 in order to find signal power S. First sub-carrier averaging unit 113 averages the power of channel gain H(k) calculated by first power calculating unit 112 by the number K of sub-carriers to find signal power S. Signal power S is represented by the following Equation (2):
      [          Equation      ⁢                          ⁢              (        2        )              ]                                S          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                      H                    ⁡                                          (                      k                      )                                                                                        2                                                                          (          2          )                    
Next, pilot replica generator 114 generates pilot signal replica H(k)C(k) from channel gain H(k) and pilot code characteristic C(k) in order to find interference power I. Here, pilot code characteristic C(k) has been previously set in the reception quality measuring apparatus. Subtractor 115 subtracts pilot signal replica H(k)C(k) generated by pilot replica generator 114 from DFT signal R(k) in the frequency domain output from reception filter 4 to output an interference signal. Second power calculation unit 116 calculates the power of the interference signal output from subtractor 115. Second sub-carrier averaging unit 117 averages the power of the interference signal calculated by second power calculation unit 116 by the number K of sub-carriers to find interference power I. Interference power I is represented by the following Equation (3):
      [          Equation      ⁢                          ⁢              (        3        )              ]                                I          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                                            R                      ⁡                                              (                        k                        )                                                              -                                                                  H                        ⁡                                                  (                          k                          )                                                                    ⁢                                              C                        ⁡                                                  (                          k                          )                                                                                                                                      2                                                                          (          3          )                    
Interference power I can also be found by using channel gain H′(k) before noise suppression instead of reception signal R(k) in Equation (3) and using channel gain H(k) instead of pilot signal replica H(k)C(k). In this event, interference power I is represented by the following Equation (4):
      [          Equation      ⁢                          ⁢              (        4        )              ]                                I          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                                                                    H                        ′                                            ⁡                                              (                        k                        )                                                              -                                          H                      ⁡                                              (                        k                        )                                                                                                              2                                                                          (          4          )                    
The principle of SIR measurement by Equation (4) is similar to Equation (3).
Subsequently, divider 118 divides signal power S found by first sub-carrier averaging unit 113 by interference power I found by second sub-carrier averaging unit 117 to calculate SIR before equalization.
FIG. 4 shows an exemplary configuration of a conventional reception quality measuring apparatus which is incorporated in the receiver shown in FIG. 1 as reception quality measuring apparatus 9. This reception quality measuring apparatus measures SIR after equalization.
The conventional reception quality measuring apparatus shown in FIG. 4 comprises multiplier 111, first power calculation unit 112, first sub-carrier averaging unit 113, pilot signal replica generator 114, subtractor 115, second power calculation unit 116, second sub-carrier averaging unit 117, and divider 118. As compared with the reception quality measuring apparatus shown in FIG. 3, this reception quality measuring apparatus uses a similar measuring principle though different inputs are used in the calculation of signal power S and interference power I.
In the following, a description will be given of the operation of the conventional reception quality measuring apparatus shown in FIG. 4.
First, multiplier 111 multiplies channel gain H(k) found by channel estimator 5 by equalization weight W(k) (1≦k≦K) found by weight calculation unit 6 to find channel gain W(k)H(k) after equalization in order to find signal power S. First power calculation unit 112 calculates the power of channel gain W(k)H(k) after equalization found by multiplier 111. First sub-carrier averaging unit 113 averages the power of channel gain W(k)H(k) after equalization found by first power calculation unit 112 by the number K of sub-carriers to find signal power S. Signal power S is represented by the following Equation (5):
      [          Equation      ⁢                          ⁢              (        5        )              ]                                S          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                                            W                      ⁡                                              (                        k                        )                                                              ⁢                                          H                      ⁡                                              (                        k                        )                                                                                                              2                                                                          (          5          )                    
Next, pilot replica generator 114 generates pilot signal replica W(k)H(k)C(k) from channel gain W(k)H(k) after equalization and pilot code characteristic C(k) in order to find interference power I. Here, pilot code characteristic C(k) has been previously set in the reception quality measuring apparatus. Subtractor 115 subtracts pilot signal replica W(k)H(k)C(k) generated by pilot replica generator 114 from equalized signal REQ(k) (1≦k≦K) in the frequency domain output from frequency domain equalizer 7 to output an interference signal. Second power calculation unit 116 calculates the power of the interference signal output from subtractor 115. Second sub-carrier averaging unit 117 averages the power of the interference signal calculated by second power calculation unit 116 by the number K of sub-carriers to find interference power I. Interference power I is represented by the following Equation (6):
      [          Equation      ⁢                          ⁢              (        6        )              ]                                I          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                                                                    R                        EQ                                            ⁡                                              (                        k                        )                                                              -                                                                  W                        ⁡                                                  (                          k                          )                                                                    ⁢                                              H                        ⁡                                                  (                          k                          )                                                                    ⁢                                              C                        ⁡                                                  (                          k                          )                                                                                                                                      2                                                                          (          6          )                    
Interference power I can also be found by using channel gain W(k)H′(k) after equalization before noise suppression instead of Equalized signal REQ(k) in Equation (6), and by using channel gain W(k)H(k) after equalization instead of pilot signal replica W(k)H(k)C(k). In this event, interference power I is represented by the following Equation (7):
      [          Equation      ⁢                          ⁢              (        7        )              ]                                I          =                                    1              K                        ⁢                                          ∑                                  k                  =                  1                                K                            ⁢                                                          ⁢                                                                                                                                    W                        ⁡                                                  (                          k                          )                                                                    ⁢                                                                        H                          ′                                                ⁡                                                  (                          k                          )                                                                                      -                                                                  W                        ⁡                                                  (                          k                          )                                                                    ⁢                                              H                        ⁡                                                  (                          k                          )                                                                                                                                      2                                                                          (          7          )                    
The principle of SIR measurement by Equation (7) is similar to Equation (6).
Subsequently, divider 118 divides signal power S found by first sub-carrier averaging unit 113 by interference power I found by second sub-carrier averaging unit 117 to calculate SIR after equalization.
However, the conventional reception quality measuring apparatus has the following challenges when it is used in the measurement of SIR after equalization as shown in FIG. 4, though no particular problems arise when it is used in the measurement of SIR before equalization as shown in FIG. 3.
Equalized signal REQ(k) includes an equalized signal component of its own signal, a residual multi-path component, and other interference (noise and other user interference).
However, the conventional reception quality measuring apparatus shown in FIG. 4 subtracts its own signal, including the residual multi-path component from equalized signal REQ(k), as represented by Equation (6), so that interference power I alone contains other interference (noise and other user interference).
Stated another way, since the conventional reception quality measuring apparatus shown in FIG. 4 regards the residual multi-path interference as signal power S, SIR that is higher than actual SIR after equalization is measured in a low SIR region, resulting in a degradation in the measurement accuracy of SIR after equalization.